Industrial Metals Supply in Burbank CA
Has 4" by 4" by 1/8 wall 6063 T5 aluminum extrusion in stock. Perfect
for plenums. $2.74 a foot and they will cut it for you.

I have a couple of Mazda throttle bodies so I decided to use
one. It is less than optimum aerodynamic wise and heavy but it
will form the base line pressure drop for the dyno engine.

More later on attaching the stock throttle body to the four inch plenum.
There is a trick to it.

Paul Lamar

Here is the problem and here is the solution. Note how the stock TB mounting
holes are incompatible with the four inch square plenum extrusion.
Simple solution; weld a 1/2 inch thick plate on the end of the plenum
and drill and tap it for 1/4 inch TB mounting screws. 

Paul Lamar

On second thought here is a much, much better solution. Where is it written
that a butterfly valve must be round? I don't know where I get these brilliant ideas.
They just occur at random. This is the easiest to build, lightest  
and cheapest to build throttle body you will ever see. Major breakthrough.

I extended the four inch square extruded tube plenum four inches further 
forward and used a square butterfly.  Further more the stock double hole butterfly 
shaft just happens to have a four inch long slot. Perfect for our use. Major 
breakthrough.

It takes about a half hour or less to make this throttle body.
The square  butterfly itself is sheared out of .049 sheet steel. What could 
be simpler to make?

If you want to get fancy you can add a couple of small instrument ball bearings
to the shaft.

Paul Lamar
 
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